Mapping two image texture on a sphere
Plug the output from the texture into the color ramp node.

That should do the trick.

Here's how it works.

When you output an image (black and white in this example), the white values stand for 1, and the black values stand for 0.
Therefore, when you plug an image texture that is black and white (like that of the lights, all the white will be 1, and all the black will be 0. In this case, the white is where there are lights, so you want to mix with an emission shader. A value of 1 (white on the texture) will mix completely, and a value of 0 (black) will not have any light.

The color ramp node, is simply for mapping how much of the texture is 1, and how much is 0. When you plug a value it, 1 (white) will be all the way to the right, and 0 all the way to the left. All in between is the values between 1 and 0. If you change the color at the right to grey, all that used to be white (1) will now be grey (0.5).

This way, if you drag the black slider almost right up to the white end, there will be no light until the texture becomes very white.

Here is an illustration.
On the right, the lesser lights are cut out, so just the powerful ones shine through. This is done by the color ramp.


Also check out Gleb's tutorial. It is a good example. Just watch how he uses it.

http://www.creativeshrimp.com/how-to-make-a-spider-web-in-blender.html

In your case, I wouldn't use it, except you may not want there to be as much light, then use it maybe. Just preference.

However, if you look, where there is not light in the texture, it is not completely black, so you may want to turn up the bottom a little bit, to crush all low values to 0. (like this) [example uses white as input for emission, not color as input, making the lights more prominent (which is what I would do when making a globe).]